Entwined tubular arrangements for heat exchangers and counterflow heat transfer systems

ABSTRACT

A counterflow heat transfer system comprises a heat exchanger and a flow controller arranged to convey a first fluid through the heat exchanger in a first flow direction and a second fluid through the heat exchanger in a second counterflow direction. The heat exchanger comprises at least one first thermally conductive tube conveying the first fluid and at least one second thermally conductive tube conveying the second fluid. The first and second tubes are wound around one another and in contact with one another in an entwined tubular arrangement.

FOREIGN PRIORITY

This application claims priority to European Patent Application No.17172935.3 filed May 25, 2017, the entire contents of which isincorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to counterflow heat transfer systems,heat exchangers and entwined tubular arrangements for heat exchangers,and additive manufacturing methods for the same.

BACKGROUND

Heat exchangers for high temperature/pressure applications are oftenmade as counter current shell-and-tube heat exchangers from materialssuch as Inconel or steel. These shell-and-tube heat exchangers contain amultitude of hollow tubes that have a hot fluid passing through them,this hot fluid being cooled by cross-flowing a colder fluid (such asair) over the tubes in an outer shell. Shell-and-tube heat exchangersare often larger than their compact cross-flow plate and fincounterparts, which are not suitable for similar applications whichshell-and-tube heat exchangers are used for.

There remains a need for improved counterflow heat transfer systems andtubular heat exchanger architectures.

SUMMARY

According to an aspect of the present disclosure there is provided acounterflow heat transfer system comprising a heat exchanger and a flowcontroller arranged to convey a first fluid through the heat exchangerin a first flow direction and a second fluid through the heat exchangerin a second counterflow direction, the heat exchanger comprising: atleast one first thermally conductive tube conveying the first fluid; andat least one second thermally conductive tube conveying the secondfluid; wherein the first and second tubes are wound around one anotherand in contact with one another in an entwined tubular arrangement.

A heat transfer system according to the present disclosure thereforetakes the counterflow of a traditional shell-and-tube heat exchanger butremoves the need for an outer shell surrounding the entwined tubulararrangements of tubes. Instead, the increased contact area between thefirst and second tubes allows for heat transfer between thecounterflowing first and second fluids. This results in a high heattransfer efficiency tubular-style heat exchanger operating as acounter-current heat transfer system. The brazed baffle plates typicallyused to hold tube bundles together in a shell-and-tube heat exchangermay no longer be needed. The system can be more compact thanconventional shell-and-tube heat exchangers. The system may have areduced volume as compared to conventional shell-and-tube heatexchangers.

In at least some examples, the flow controller is arranged to convey afirst fluid through the heat exchanger that is hotter than the secondfluid, the heat exchanger comprising a plurality of second thermallyconductive tubes conveying the second (i.e. cooler) fluid. In suchexamples, the heat exchanger may comprise one first thermally conductivetube conveying the first (i.e. hotter) fluid and at least two secondthermally conductive tubes conveying the second (i.e. cooler) fluid. Insome examples, there may be two, three, four or more second thermallyconductive tubes conveying the second (i.e. cooler) fluid for each firstthermally conductive tube conveying the first (i.e. hotter) fluid in theentwined tubular arrangement. It will be appreciated that the number oftubes conveying a given fluid at any point of time is a feature of thesystem that can be determined during use. At different times, there maybe a different number of first tubes conveying the first (i.e. hotter)fluid and second tubes conveying the second (i.e. cooler) fluid. Thus itwill be understood that, in these examples, the system has at least onemode of operation wherein there are more of the second thermallyconductive tubes conveying the second (i.e. cooler) fluid than there arefirst thermally conductive tube(s) conveying the first (i.e. hotter)fluid. Of course the system may operate in other modes of operationother times.

In addition, or alternatively, in examples where the flow controller isarranged to convey a first fluid through the heat exchanger that ishotter than the second fluid, the Applicant has recognised that the heattransfer efficiency of the counterflow system can be improved byarranging for the second thermally conductive tube(s) conveying thesecond (i.e. cooler) fluid to have a larger surface area to volume ratiothan the first thermally conductive tube(s) conveying the first (i.e.hotter) fluid. In such examples, one or more of the second thermallyconductive tube(s) conveying the second (i.e. cooler) fluid may have asmaller diameter than the first thermally conductive tube(s) conveyingthe first (i.e. hotter) fluid. In a system having a common pressure forthe first and second fluids, this also means that the second (i.e.cooler) fluid may flow faster and this also promotes heat transfer awayfrom the first (i.e. hotter) fluid.

The Applicant has realised that the tubes conventionally used to conveyfluids in a heat exchanger tend to have a round cross-section and thislimits the contact surface area for heat transfer, even when the tubesare in contact with one another in an entwined tubular arrangement. Itis desirable to increase the contact surface area.

In at least some examples, the heat exchanger comprises a thermallyconductive filler material between the first and second tubes where theyare in contact with one another. To maximise heat transfer efficiency,the filler material may be substantially the same thermally conductivematerial as the first and/or second tubes. In at least some examples thefirst and second tubes are made of the same thermally conductivematerial. The amount of thermally conductive filler material may bechosen to achieve a desired level of heat transfer efficiency.

In at least some examples, the thermally conductive filler materialforms a brazed joint between the first and second materials. The brazedjoint may be a fillet-brazed joint. The shape and/or size of the filletsmay be chosen to achieve a desired level of heat transfer efficiency.

In addition, or alternatively, in at least some examples one or more ofthe first thermally conductive tubes and/or one or more of the secondthermally conductive tubes may comprise a flattened surface area wherethe first and second tubes are in contact with one another. This helpsto increase the contact surface area for heat transfer.

In addition, or alternatively, in examples where the flow controller isarranged to convey a first fluid through the heat exchanger that ishotter than the second fluid, one or more of the second thermallyconductive tubes conveying the second (i.e. cooler) fluid may comprise anon-circular cross-section. These tubes may have a cross-sectioncomprising a circular portion and a flattened portion, and/or a circularportion and a concave portion. In at least some examples, one or more ofthe first thermally conductive tubes and/or one or more of the secondthermally conductive tubes may comprise a concave surface area where thefirst and second tubes are in contact with one another. Such tubeprofiles can help to maximise the contact surface area for transferringheat from the first fluid to the second fluid.

In a counterflow heat transfer system as disclosed above, the heatexchanger may be manufactured by any suitable process that can providethe entwined tubular arrangement of first and second tubes. Conventionalmanufacturing techniques commonly used to make tubular heat exchangers,e.g. metal extrusion or casting processes, may not be appropriate. Insome examples, the entwined tubular arrangement may be made byinvestment casting. In some other examples, the entwined tubulararrangement may be made by additive manufacturing. In particular, anadditive manufacturing (AM) technique may be used to build up athermally conductive material layer-by-layer from a Computer-AidedDesign (CAD) model. Especially when using additive manufacturing to makemetal parts, such techniques often require the use of sacrificialsupport structures to hold the parts during the manufacturing process.The support structures are built, layer by layer, simultaneously withthe object and then removed after the object is fully constructed.However, the Applicant has recognised that the support structure whichis typically removed at the end of an additive manufacturing process canprovide a heat exchanger with certain benefits if left in situ. Thus, inat least some examples the entwined tubular arrangement is at leastpartially supported by a support structure, preferably made of the samethermally conductive material as the first and second tubes. Thisstructure is useful not only to aid in heat dissipation during themanufacturing process but also to help transfer heat out of those tubesconveying a hotter fluid during subsequent use of the heat exchanger.

According to another aspect of the present disclosure there is providedan entwined tubular arrangement for a heat exchanger, the arrangementcomprising: at least one first thermally conductive tube for conveying afirst fluid; at least one second thermally conductive tube for conveyinga second fluid; wherein the first and second tubes are wound around oneanother and in contact with one another in an entwined tubulararrangement; and wherein the entwined tubular arrangement is supportedby a support structure, preferably made of the same thermally conductivematerial as the first and second tubes.

An entwined tubular arrangement as disclosed herein may be used in acounterflow heat exchanger or a cross-flow heat exchanger, for instancethe shell-and-tube type of heat exchanger. Examples of such an entwinedtubular arrangement may independently include any of the features, takenalone or in any combination, already disclosed above in relation to acounterflow heat transfer system. Such features may relate to, forexample, the number/diameter/cross-section/surface area of the firstand/or second thermally conductive tubes in the arrangement, and anythermally conductive filler material between the first and second tubeswhere they are in contact with one another.

It will be appreciated that the support structure can aid in heatdissipation and structural support during manufacture of the entwinedtubular arrangement, and also provide an additional means of heatdissipation when the entwined tubular arrangement is used in a heatexchanger. The support structure may also provide the entwined tubulararrangement with a mechanical support to mitigate against unwantedmovement or vibration during use.

In additive manufacturing processes it is known to use thin,scaffold-like structures, or structures with small pointed teeth, tominimise the amount of part contact so that the support structures canbe broken away from the manufactured part easily using hand tools. TheApplicant has realised that a lattice-type support structure can alsoprovide benefits when retained in an entwined tubular arrangement for aheat exchanger. The support structure may therefore comprise a latticesupport structure, ideally a lattice support structure with very lowvolume fraction.

In some examples, a third fluid may be conveyed through the latticesupport structure to assist in heat transfer. Thus, in examples of acounterflow heat transfer system as disclosed above, the flow controllermay be arranged to convey a third fluid through the lattice supportstructure. The third fluid may be liquid or gas. In at least someexamples, the flow controller may be arranged to convey first and secondcounterflow liquids through the entwined tubular arrangement and acooling air flow through the lattice support structure.

In some other examples, in addition or alternatively, the latticesupport structure may be at least partially filled with a thermallyinsulative material. This can result in minimal heat energy loss to thesurrounding support structure and avoid radiative losses from theentwined tubular arrangement. For example, the lattice support structuremay be filled with a lightweight polymer resin in a post-manufacturingprocess, the thermally insulative polymer resin acting to force heattransfer in the desired direction (i.e. from hotter to cooler fluid) inthe entwined tubular arrangement.

For the support structure to be made of the same thermally conductivematerial as the first and second tubes, investment casting or additivemanufacturing may be used. However, additive manufacturing techniquesmay be particularly well-suited for making entwined tube bundles thatcould not be made with conventional manufacturing methods. Thus, invarious examples, the entwined tubular arrangement is formed by anadditive manufacturing technique used to build up the thermallyconductive material layer-by-layer from a Computer-Aided Design (CAD)model.

According to a further aspect of the present disclosure there isprovided an additive manufacturing method of making an entwined tubulararrangement for a heat exchanger, comprising: using an additivemanufacturing technique to build up one or more thermally conductivematerials layer-by-layer from a Computer-Aided Design (CAD) model;building a support structure and an entwined tubular arrangement out ofthe thermally conductive material(s), wherein the entwined tubulararrangement comprises first and second tubes wound around one anotherand in contact with one another; and retaining at least some of thesupport structure after the entwined tubular arrangement has been built.

It will be understood that at least some of the support structure,possibly all of the support structure, is retained after building theentwined tubular arrangement. This is unusual, as in additivemanufacturing techniques the support structure that is built to hold theparts during manufacture is usually sacrificed at the end of themanufacturing process. Preferably the support structure and the entwinedtubular arrangement are built out of the same thermally conductivematerial.

The thermally conductive material(s) may be one or more of: ceramic,metal matrix composite, alloy or metal. Any suitable additivemanufacturing (AM) or additive layer manufacturing (ALM) technique maybe used. Some exemplary techniques include powder bed fusion (PBF),Selective Laser Melting (SLM), Selective Laser Melting (SLM), DirectMetal Laser Sintering (DMLS), Selective Laser Sintering (SLS), andelectron beam melting (EBM) processes. Powder Bed Laser Fusion (PBF) orElectron Beam Melting (EBM) may be particularly suitable.

In examples according to any of the aspects of the disclosure above, oneor more of the first and/or second thermally conductive tubes maycomprise fins on an inside or outside surface. Such fins may furtherassist in achieving high heat transfer efficiency.

In examples according to any of the aspects of the disclosure above, theentwined tubular arrangement may take any suitable form. The first andsecond tubes may be wound around one another in a helical or non-helicalarrangement. In some examples the first and second tubes are helicallywound around a common axis in contact with one another in the entwinedtubular arrangement.

In examples according to any of the aspects of the disclosure above, thethermally conductive material of the first and/or second tubes may be ametal (e.g. steel), alloy (e.g. Inconel or Haynes 282), metal matrixcomposite or ceramic. The thermally conductive filler material, whereprovided, may be the same or different thermally conductive material ofchosen from a metal, alloy, metal matrix composite or ceramic. Thethermally conductive material(s) may be chosen for compatibility withhigher temperature and/or pressure aerospace applications.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more non-limiting examples will now be described, by way ofexample only, and with reference to the accompanying figures, in which:

FIG. 1 is a schematic overview of an entwined tubular arrangement;

FIG. 2 is a schematic cross-sectional view of two tubes in an exemplaryentwined tubular arrangement;

FIG. 3 is a schematic cross-sectional view of two tubes in anotherexemplary entwined tubular arrangement; and

FIGS. 4a and 4b are schematic cross-sectional views of multiplesecondary tubes in an entwined arrangement with a primary tube.

DETAILED DESCRIPTION

There is seen in FIG. 1 an overview of an entwined tubular arrangement 2comprising a first thermally conductive e.g. metallic tube 4 conveying ahot fluid in the flow direction of the arrow A. The primary tube 4 ishelically wound around a common axis in contact with two thermallyconductive e.g. metallic secondary tubes 6 a, 6 b conveying a coolingfluid in the counterflow direction of the arrow B. The double-headedarrows C indicate heat transfer from hot to cold. The counterflow offluids being conveyed through the first and second tubes 4, 6 a, 6 b ina heat exchanger is controlled by a flow controller (not shown) to forma counterflow heat transfer system.

FIG. 2 illustrates a first example of an entwined tubular arrangement20, using fillets to maximise heat transfer surface area. In thisarrangement 20, a first thermally conductive e.g. metallic tube 24 and asecond thermally conductive e.g. metallic tube 26 are helically woundaround a common axis in contact with one another. One or more additionalsecond tubes 26 may also be included, for example as generally seen inFIGS. 4a and 4b . In a heat exchanger, the first tube 24 may be used toconvey a hot fluid that requires cooling and the second tube(s) 26 maybe used to convey a cooling fluid in a counterflow direction. The secondtube 26 has a smaller diameter than the first tube 24. Fillets 28 a, 28b are placed in the area where the tubes 24, 26 are in contact with oneanother, to increase the heat transfer surface area. Both the first andsecond tubes 24, 26 optionally include fins 30 extending inwardly fromthe area where the tubes are in contact with one another, to provide asecondary heat transfer surface. Such an entwined tubular arrangement 20may be made by additive layer manufacturing techniques.

FIG. 3 illustrates a second example of an entwined tubular arrangement40 comprising a first thermally conductive e.g. metallic tube 44 and asecond thermally conductive e.g. metallic tube 46 helically wound arounda common axis in contact with one another. One or more additional secondtubes 46 may also be included, for example as generally seen in FIGS. 4aand 4b . In a heat exchanger, the first tube 44 may be used to convey ahot fluid that requires cooling and the second tube(s) 46 may be used toconvey a cooling fluid in a counterflow direction. The second tube 46has a smaller diameter than the first tube 44. The second tube 46 has aconcave surface area 48 in contact with the round first tube 44. Theentwined tubular arrangement 40 comprises a lattice support structure50, preferably made of the same thermally conductive material as thefirst and second tubes 44, 46. The lattice support structure 50 wasbuilt when an additive manufacturing technique was used to build up thethermally conductive material layer-by-layer from a Computer-AidedDesign (CAD) model, and subsequently retained post-manufacture. Thelattice support structure 50 is optionally filled with a thermallyinsulative (e.g. polymer or resin) material 52 that can act as aninsulating barrier, forcing the heat transfer in the direction desiredfrom the first tube 44 to the second tube(s) 46.

FIGS. 4a and 4b illustrate some further examples of entwined tubulararrangements 60, 80 that may be used in conjunction with any of thepreviously described examples.

In FIG. 4a , the entwined tubular arrangement 60 comprises a firstthermally conductive e.g. metallic tube 64 and two secondary thermallyconductive e.g. metallic tubes 66 that are all helically wound around acommon axis in contact with one another. The entwined tubulararrangement 60 comprises a lattice support structure 70, preferably madeof the same thermally conductive material as the tubes 64, 66. Thelattice support structure 70 is optionally filled with a thermallyinsulative (e.g. polymer or resin) material. In a heat exchanger, theprimary tube 64 may be used to convey a hot fluid that requires coolingand the secondary tubes 66 may be used to convey a cooling fluid in acounterflow direction. The secondary tubes 66 have a smaller diameterthan the primary tube 64. The small arrows depict how heat may betransferred from the primary tube 64 to the secondary tubes 66 throughsurface contact.

In FIG. 4b , the entwined tubular arrangement 80 comprises a firstthermally conductive e.g. metallic tube 84 and three secondary thermallyconductive e.g. metallic tubes 86 that are all helically wound around acommon axis in contact with one another. The entwined tubulararrangement 80 comprises a lattice support structure 90, preferably madeof the same thermally conductive material as the tubes 84, 86. Thelattice support structure 90 is optionally filled with a thermallyinsulative (e.g. polymer or resin) material. In a heat exchanger, theprimary tube 84 may be used to convey a hot fluid that requires coolingand the secondary tubes 86 may be used to convey a cooling fluid in acounterflow direction. The secondary tubes 86 have a smaller diameterthan the primary tube 84. The small arrows depict how heat may betransferred from the primary tube 84 to the secondary tubes 86 throughsurface contact.

Although not shown in FIGS. 4a and 4b , optionally there may be filletsplaced in the area where the secondary tubes 66, 86 are in contact withthe primary tube 64, 84 to increase the heat transfer surface area. Inaddition, or alternatively, any of the tubes 64, 66, 84, 86 mayoptionally include fins extending inwardly from the area where the tubesare in contact with one another, to provide a secondary heat transfersurface.

While currently available additive layer manufacturing techniquestypically use the same material throughout the layer building process,it is envisaged that a different thermally conductive (e.g. metallic)material may be used for the lattice support structure 50, 70, 90. Thismay be exploited, for example, to optimise the heat transfer propertiesof the lattice support structure as compared to the tubes.

1. A counterflow heat transfer system comprising a heat exchanger and aflow controller arranged to convey a first fluid through the heatexchanger in a first flow direction and a second fluid through the heatexchanger in a second counterflow direction, the heat exchangercomprising: at least one first thermally conductive tube conveying thefirst fluid; and at least one second thermally conductive tube conveyingthe second fluid; wherein the first and second tubes are wound aroundone another and in contact with one another in an entwined tubulararrangement.
 2. The counterflow heat transfer system according to claim1, wherein the flow controller is arranged to convey a first fluidthrough the heat exchanger that is hotter than the second fluid, theheat exchanger comprising a plurality of second thermally conductivetubes conveying the second fluid.
 3. The counterflow heat transfersystem according to claim 1, wherein one or more of the second thermallyconductive tube(s) conveying the second fluid may have a smallerdiameter than the first thermally conductive tube(s) conveying the firstfluid.
 4. The counterflow heat transfer system according to claim 1,wherein one or more of the second thermally conductive tubes conveyingthe second fluid comprise a non-circular cross-section.
 5. Thecounterflow heat transfer system according to claim 1, wherein the heatexchanger comprises a thermally conductive filler material between thefirst and second tubes where they are in contact with one another. 6.The counterflow heat transfer system according to claim 5, wherein thethermally conductive filler material forms a brazed joint between thefirst and second materials.
 7. The counterflow heat transfer systemaccording to claim 1, wherein one or more of the first thermallyconductive tubes and/or one or more of the second thermally conductivetubes comprise a flattened or concave surface area where the first andsecond tubes are in contact with one another.
 8. The counterflow heattransfer system according to claim 1, wherein the entwined tubulararrangement is made by additive manufacturing.
 9. The counterflow heattransfer system according to claim 1, wherein the entwined tubulararrangement is at least partially supported by a support structure madeof the same thermally conductive material as the first and second tubes.10. An entwined tubular arrangement for a heat exchanger, thearrangement comprising: at least one first thermally conductive tube forconveying a first fluid; at least one second thermally conductive tubefor conveying a second fluid; wherein the first and second tubes arewound around one another and in contact with one another in an entwinedtubular arrangement; and wherein the entwined tubular arrangement issupported by a support structure made of the same thermally conductivematerial as the first and second tubes.
 11. The entwined tubulararrangement according to claim 10, wherein the support structurecomprises a lattice support structure.
 12. The entwined tubulararrangement according to claim 11, wherein a flow controller is arrangedto convey a third fluid through the lattice support structure.
 13. Theentwined tubular arrangement according to claim 11, wherein the latticesupport structure is at least partially filled with a thermallyinsulative material.
 14. The entwined tubular arrangement according toany preceding claim 11, wherein the entwined tubular arrangement isformed by an additive manufacturing technique used to build up thethermally conductive material layer-by-layer from a Computer-AidedDesign (CAD) model.
 15. An additive manufacturing method of making anentwined tubular arrangement for a heat exchanger, comprising: using anadditive manufacturing technique to build up one or more thermallyconductive materials layer-by-layer from a Computer-Aided Design (CAD)model; building a support structure and an entwined tubular arrangementout of the thermally conductive material(s), wherein the entwinedtubular arrangement comprises first and second tubes wound around oneanother and in contact with one another; and retaining the supportstructure after the entwined tubular arrangement has been built.